This invention relates to solidification processing of metallic alloys, and, more particularly, to semi-solid processing of metallic alloys.
The casting of a metal into a useful shape involves heating the metal to a temperature above its melting point, placing the molten metal into a form termed a mold, and cooling the metal to a temperature below its melting point. The metal solidifies in the shape defined by the mold, and is thereafter removed from the mold. Within these general guidelines, a wide variety of casting technologies are known.
When most metallic alloys are cooled from the molten state, they do not solidify at a single temperature, but over a temperature range. As the metal is cooled, it first reaches a liquidus temperature at which the alloy begins to freeze. As the temperature is further reduced, an increasing fraction of the metal becomes solid, until the metal is entirely solid below a solidus temperature.
In conventional casting practice, the metal is cooled from the molten state above the liquidus temperature to the solid state below the solidus temperature, without being held at a temperature between the liquidus temperature and the solidus temperature. However, it is known to cool the metal to a semi-solid temperature range between the liquidus temperature and the solidus temperature and hold the metal at that temperature, so that the metal is in a semi-solid state. Alternatively, the metal may be heated from a temperature below the solidus temperature to the semi-solid temperature range between the liquidus temperature and the solidus temperature. By whatever path the metal reaches this semi-solid temperature range, the semi-solid material is then processed to produce a structure of solid globules in a liquid matrix. This process may involve intensive stirring, but if suitable conditions are achieved to give many crystallization nuclei (for example by rapid cooling or using suitable grain refinement techniques) the process may involve only an aging step. The semi-solid mixture is then forced into a mold while in this semi-solid state, typically by die casting.
In the conventional semi-solid casting technique, careful control is required over the heating and cooling parameters, specifically the holding temperature at which the processing apparatus is maintained. The present inventors have realized that for commercial purposes the conventional approach is confined to use with alloys having a low rate of increase of the fraction of solids with decreasing temperature, at the semi-solid processing temperature. Consequently, many alloys are excluded from practical commercial semi-solid processing, unless a high degree of control on temperature (requiring expensive equipment) is achieved. This high degree of control is not possible or not practical for many commercial semi-solid casting operations
Accordingly, there is a need for an improved approach to the semi-solid casting of metallic alloys, which is less restrictive on processing parameters and produces a better-quality final product. The present invention fulfills this need, and further provides related advantages.